In recent years, a four-layer antireflection coating disclosed by Rock in U.S. Pat. No. 3,432,225, has found widespread use in commercial optical coating practice. The coating comprises first and second layers of a low refractive index material, for example magnesium fluoride, and second and fourth layers formed from a high refractive index material for example zirconium dioxide. The layers are numbered, here, in order, beginning with the layer furthest from a substrate on which the coating is deposited. The first layer has an optical thickness of about one-quarter wavelength of visible light, the second layer has an optical thickness of about one-half wavelength of visible light, and the third and fourth layers have a combined optical thickness of between about one-tenth and one-quarter wavelength of visible light.
An advantage of the coating or layer system is that its optical performance is not entirely dependent on the existence of materials having a specific value of refractive index. For any given value of refractive index of the substrate, however, there are preferred values of refractive index of the first and second films which will provide optimum reflection reduction for the layer system, in particular, the bandwidth over which the layer system is effective in reducing reflection.
Sputtering technology has now been advanced to the stage where in-line sputter coating machines may be used to deposit antireflection coatings such as the coating of Rock. A significant current limitation of the technology is that, of materials which may be practically deposited by sputtering, the material having the lowest refractive index is silicon dioxide, which has a refractive index of about 1.46 at a wavelength of about 520 nm. In a Rock type layer system having a first layer of silicon dioxide, the second layer preferably has a refractive index of about 2.35 for visible light. Titanium dioxide is such a material, but it has a low sputtering rate compared with silicon dioxide. Titanium oxide is also not electrically-conductive. High sputtering rates are essential in reducing production costs for in in-line sputtered coatings. Electrically-conductive anti-reflection coatings are finding increasing application, for example, in protective anti-glare screens for video display units.
Metal oxide materials which sputter at high rates, or which can be made to be electrically-conductive, include zinc oxide, indium oxide, tin oxide and the like which as a group have a refractive index between about 1.9 and 2.1 at a wavelength of about 520 nm. Using these materials as the second layer in a Rock type anti-reflection layer system having a silicon dioxide first layer provides an anti-reflection performance which is generally unacceptable, even in exchange for reduced cost or high electrical-conductivity.
In U.S Pat. No. 5,105,310, Dickey discloses a variation of the layer system of Rock which is specifically designed for high-rate sputter-deposition. In a simplest five-layer embodiment of Dickey's layer system, the second layer of Rock is subdivided into second and third layers. Fifth and sixth layers correspond generally to the above described fourth and fifth layers of Rock. In other examples, the second layer of Rock is replaced by second third and fourth layers. In all examples disclosed by Dickey, the second layer of a group of two or more layers replacing the second layer of Rock must have the highest refractive index of the group, and further, must have a refractive index greater than 2.2. The remaining layers may be formed from zinc oxide or some other material which has a lower refractive index but a higher sputtering rate than the second layer. Dickey teaches that the antireflection performance reduction provided by the substitution is acceptable in view of the cost benefit.
In Dickey, the reflection compromise appears to be achieved by providing a spectral response curve having a reduced bandwidth compared with an optimum Rock system from which it is derived. Although, in photopic terms, the compromise appears acceptable, the reduced bandwidth provides relatively high reflection at the violet and red spectral extremes. Further, in the system of Dickey, a the maximum thickness of material having a refractive index of about 2.0 is less than one-half wavelength of visible light. This maximum thickness sets an upper limit on the conductivity which may be obtained if the refractive index 2.0 material is an electrically-conductive metal oxide.